coronal abundances determined from energetic particles

Adv. SpaceRes. Vol. 15,No. 7,pp.(7)41-(7)51,1995

(\J)

Pergamon

1995 COSPAR

Peg

Printed in Great Britain.

0273-1 177/95$9.50+ 0.00

0273-1177(94)00018-2

CORONAL ABUNDANCES DETERMINED FROM ENERGETIC PARTICLES D. V. Reames NASA Goddard Space Flight Center, Code 661, Greenbelt, MD 20771, U.S.A.

ABSTRACT Solar energetic particles (SEPs) provide a measurement of coronal element abundances that is highly independent of the ionization states and temperature of the ions in the source plasma. The most complete measurements come from large 'gradual' events where ambient coronal plasma is swept up by the expanding shock wave from a coronal mass ejection. Particles from 'impulsive' flares, have a pattern of acceleration-induced enhancements superimposed on the coronal abundances. Particles accelerated from high-speed solar wind streams at corotating shocks show a different abundance pattern corresponding to material from coronal holes. Large variations in He/O in coronal material are seen for both gradual and impulsive-flare events but other abundance ratios, such as Mg/Ne, are remarkably constant. SEP measurements now include hundreds of events spanning 15 years of high-quality measurement.

INTRODUCTION Energetic particles, associated with large eruptive solar events, have provided both the earliest and the most complete information on element abundances in the solar corona. The first observation of energetic heavy ions from the Sun was made over 30 years ago by Fichtel and Guss /1/ using nuclear emulsion detectors flown on sounding rockets, and these observations were extended up to the element Fe a few years later /2/. During the 1970's instruments on satellites led to substantial improvements in the both the element resolution and statistics of the measurements. In a classic review of these measurements of solar-energetic-particle (SEP) abundances, Meyer /3/ found that the abundances, averaged over many events, showed a twolevel dependence upon the first ionization potential (FIP) of the element, indicating an ion-neutral fractionation as material is transported up to the corona. Superimposed on the average abundances were eventto-event abundance variations that were increasingly large for heavier elements. This 'mass bias' resulted from a dependence of the acceleration process on the charge to mass ratio, Q/A, that the ions have when they are accelerated from the ambient coronal plasma at a temperature of -2 MK /4, 5/. As Meyer /3/ recognized, there was also a separate class of SEP events, called 'He-rich events, that have -1000-fold enhancements in 3 He/4He and -10-fold enhancements in Fe/O. In these events, large enhancements arising from wave-particle interactions during acceleration prevented an easy determination of the underlying abundances in the source plasma. During the last several years there has been a substantial revision in our understanding of the origin and

acceleration of particles at the Sun /6/. We have found that most of the large classic 'gradual' SEP events are not associated with solar flares but, rather, with coronal mass ejections (CMEs). Particles are accelerated from the ambient plasma at the shock wave driven ahead of the CME in these events. It is the 3 He-rich events that come from impulsive flares. In impulsive flares, intense beams of electrons generate electromagnetic ion cyclotron (EMIC) waves below the proton gyrofrequency where they are resonantly absorbed by 3 He, enhancing its abundance in the accelerated particles by several orders of magnitude. A new understanding of the physics of acceleration has helped us decouple the effects of acceleration and access the abundances in the source plasma in both impulsive and gradual events. (7)41

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An entirely different population of energetic particles is accelerated at shock waves produced outside 1 AU at corotating interaction regions (CIRs) where high-speed solar wind streams overtake low-speed streams. These regions are especially prominent during solar minimum when they often persist for many solar rotations. Particles accelerated from the high-speed stream show a different dependence of the abundances on FIP indicating a difference in the ion-neutral fractionation process beneath coronal holes /7, 8/. Thus, three distinct populations of energetic particles have been identified that come from different coronal regions: 1) In the classic large gradual events, particles are accelerated by a shock wave driven by a coronal mass ejection (CME). As the shock traverses the corona and solar wind, it samples elements in a reasonably democratic fashion, on average, allowing us to determine fairly complete abundances for Z